LCD driving system with low power requirements

ABSTRACT

Two separate power supplies are employed to generate electrical potentials for driving row and column electrodes of LCDs. One or more energy storage devices are preferably used together with the two power supplies for generating such potentials. In the first phase, the energy storage devices are charged and in the second phase, such devices and the power supplies are employed to generate the appropriate potentials for driving the row and column electrodes. Such schemes permit the column electrodes to be driven through a voltage range much smaller than the convention IAPT driving schemes and vastly reduces the power consumption by the driver. The total voltage dynamic range experienced by the driver circuit is comparable to the IAPT driving scheme.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 09/561,737, filed Apr. 28, 2000, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] This invention relates in general to liquid crystal displays (LCDs) and, in particular, to a low power driving scheme for LCDs.

[0003]FIG. 1 is a schematic view of a LCD panel and its n row electrodes labeled COM 1, . . . COM n in FIG. 1, and k column electrodes shown as vertical rectangles labeled SEG 1˜SEGk in FIG. 1. Not shown in FIG. 1 (to simplify the figure) is a layer of liquid crystal material between the row and column electrodes. Each row electrode will overlap a column electrode when viewed in a viewing direction, where the overlapping portion of the two overlapping electrodes will define a pixel of the LCD panel. When an appropriate voltage is applied across a particular row and a particular column electrode, a portion of the liquid crystal layer at the pixel between the overlapping row and column electrodes controls the light transmission or reflective properties of such portion.

[0004]FIG. 2a is a graphical illustration of the Improved Alto-Pleshko (IAPT) waveform for the row (or COM) electrodes and column (or SEG) electrodes. FIG. 2b is a graphical plot of the conventional Alto-Pleshko driving waveform for row (COM) and column (SEG) electrodes. In FIGS. 2a and 2 b, voltages labeled V_(COM) or variations thereof indicate voltage waveforms that are applied to the row electrodes and voltages labeled V_(SEG) or variations thereof indicate voltage waveforms applied to column electrodes. The driving waveforms in FIGS. 2a and 2 b are conventional. Referring to FIG. 1 and FIG. 2a, a typical configuration of passive LCD and a conventional driving waveform is illustrated. As demonstrated in FIG. 1, the ith row electrode is connected to a node at voltage V_(COMi), on one side, and the jth column electrode is connected to a node at voltage V_(SEGj) on the other side. In FIG. 2a, where vertical axis is voltage, and the horizontal axis time, the data signals V_(SEGj) are also drawn as overlapped shaded region over V_(COMi) signal to illustrate relative relationships between these two sets of signals.

[0005] The driving waveform demonstrated in FIG. 2a, is known as Improved Alto-Pleshko driving method (Improved APT, or IAPT for brief). The main characteristic is that the COM scanning pulses are “folded” such that the driving total voltage dynamic range is reduced as compared to the plain APT, as show in FIG. 2b. This reduced voltage range is considered to be advantageous in the conventional design technique used in CMOS integrated driver IC, where low MOS transistor break down voltage (caused by thin gate oxide used in fine gate geometry circuits and devices) would otherwise make the circuit design very difficult.

[0006] From the waveform of these signals it is observed that although the IAPT driving method reduces the voltage dynamic range for the drivers, but the power is increased as a consequence. This is because the LCD is a pure AC device and the capacitive load on column (SEG) electrodes can be quite significant. However, the current driving the column (SEG) electrodes also need to flow through the entire voltage range, although the column (SEG) electrode voltage swing, V6-V4, or V3-V1 as shown in FIG. 2a (these two value needs to be the same), as measured from “majority” COM electrode voltage (which is substantially the same as the non-scanning voltage V₅ in Field 2×N and V₂ in Field 2×N+1) is far smaller than the total supply voltage used in a conventional IAPT driving scheme (V6-V1 in FIG. 2a.).

[0007] According to conventional design principal, the ratio between (V6-V1) and (V3-V1) can be estimated roughly by $\sqrt{\frac{{Mux} + 1}{2}},$

[0008] where Mux is the multiplexing rate (or duty factor), which is determined by the number of row/COM electrodes. Using this formula, for a moderate sized LCD of 81 rows (with 81 row/COM electrodes), the above ratio is 5×, and therefore, the power wasted for driving SEG electrodes (which is proportional to voltage V, assuming current I stays unchanged) can be as high as 80%.

[0009] As there will be only one COM electrode scanning while all SEG electrodes can change at each row scanning period, SEG/column electrode capacitive loading current can be more than ten times higher than COM/row electrode loading current. This obviously makes the low utilization of supplied SEG/column power very undesirable.

[0010] One of the most frequently heard complaints from users of portable computers, cellular phones and personal digital assistants is that these devices consume too much power so that one has to constantly change batteries, which is inconvenient. It is, therefore, desirable to provide a power saving scheme for driving LCD displays, especially for displays used in such portable devices.

[0011] Another problem encountered in conventional LCD is crosstalk caused by its driving circuit design. Many passive LCD devices are driven by means of Class B bias circuits. In such circuits, to minimize power consumption, both N and P transistors are in the OFF state when the output of the circuit is not used to drive electrodes. Therefore, when the Class B bias circuit is used to drive the column electrodes to target voltage values, as the electrical potential of the driven electrode approaches the target value, the output error is small so that the output stage of conventional operational amplifier driver circuits moves towards Class B bias, where both N and P type transistors are in the OFF state and there is therefore very weak driving power. This causes the driver circuit to be in high impedance state due to the high RC value of the load driven by the drive circuit. Therefore, the residual error (the difference between the voltage across the row and column electrodes at the pixel addressed and the target value) tends to persist for a long time through a number of row-scanning cycles, thereby causing crosstalk. In this context, a row scanning cycle is the time period taken for applying electrical potentials to cause pixels overlapping a row electrode to have the opportunity to change state.

[0012] It is therefore desirable to provide an improved LCD driving scheme where the above-described crosstalk problem is reduced or minimized.

SUMMARY OF THE INVENTION

[0013] This invention is based on the recognition that by using two separate power supplies to generate suitable electrical potentials for driving the row and column electrodes, the current driving the column (SEG) electrodes does not need to flow through the total supply voltage (V6-V1 in FIG. 2A), so that the power consumed by the LCD can be significantly reduced. Preferably, one of the electrical potentials caused to be generated using the two power sources and applied to the row and column electrodes floats with the voltage supplied by one of the power sources. This allows the dynamic range of electrical potentials driving the LCD to be reduced, thereby also allowing the size of semiconductor devices in a power circuit to be reduced. This enables cheaper driver systems to be manufactured for driving LCD displays.

[0014] Since the LCD is a pure AC device, signal pulses driving the LCD will be higher than a reference voltage such as ground in some field addressing cycles (e.g. positive going pulses) and lower than the reference voltage (e.g. negative going pulses) in other field addressing cycles. In one embodiment, by providing a voltage differential which is in turn used for generating both types of pulses, this ensures that no significant DC offset would develop across LCD pixels that can cause ionization of the liquid crystal material and damage the LCD permanently. In some embodiments, such voltage differential may be supplied directly by a voltage supply.

[0015] Preferably, an energy storage device is used in the low power driving scheme of this invention for generating electrical potentials suitable for driving the row and column electrodes. In a first phase of the operation of the scheme, one or more energy storage devices are charged, and in a second later phase, the energy stored is then used to drive the row and/or column electrodes. In some embodiments, power from the power sources may be used to both charge the energy storage devices as well as to drive row and/or column electrodes during the first phase.

[0016] In embodiments where one or more energy storage devices are employed, some of the current employed for driving the column electrodes in a field addressing cycle may be reused in a subsequent cycle for driving column electrodes.

[0017] Another aspect of the invention is based on the observation that, by using energy storage devices such as capacitors to drive the column electrodes, the above-described problems encountered with Class B bias circuits can be avoided altogether. Thus, if the energy storage device chosen has a much higher capacitance than the load it is driving, such as the capacitive load of the LCD pixels, it is possible to drive the column electrodes to a voltage close to or substantially at a target value within a row-scanning cycle, so that the above-described crosstalk problem associated with conventional drivers can be alleviated. In the preferred embodiment, two different energy storage devices may be used, with the first device used to drive the column electrode to a voltage close to but not at the target value, and the second device used to drive the column electrode then to substantially the target value, all within the same row-scanning cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic view of a LCD panel and its row electrodes labeled COM 1, . . . COM n in FIG. 1, and column electrodes shown as vertical rectangles labeled SEG 1˜SEGk in FIG. 1.

[0019]FIG. 2a is a graphical illustration of the conventional Improved AltoPleshko (IAPT) waveform for the row or COM electrodes and column or SEG electrodes.

[0020]FIG. 2b is a graphical plot of the conventional Alto-Pleshko driving waveform for row (COM) and column (SEG) electrodes.

[0021]FIG. 3a is a schematic circuit diagram of a power supply circuit generating suitable electrical potentials for driving the row and column electrodes of a LCD to illustrate one embodiment of the invention.

[0022]FIGS. 3b-3 d are schematic circuit diagrams of portions of a power supply circuit generating electrical potentials suitable for driving the row and column electrodes of an LCD display to illustrate further embodiments.

[0023]FIG. 4a is a schematic circuit diagram of a power supply circuit for generating electrical potentials for driving a LCD with electrical potentials that float with a voltage supplied by a power supply to illustrate yet another embodiment of the invention.

[0024]FIGS. 4b and 4 c are graphical plots of signal waveforms employed in the embodiment of FIG. 4a.

[0025]FIGS. 5a and 5 b together illustrate a power supply circuit for generating electrical potentials suitable for driving the row and column electrodes of an LCD to illustrate a preferred embodiment of the invention.

[0026]FIG. 6a is a schematic circuit diagram of a power supply circuit for generating electrical potentials suitable for driving the column electrodes of a LCD to illustrate yet another aspect of the invention directed to the reduction of crosstalk.

[0027]FIG. 6b is a timing diagram of switching circuit waveforms and voltage waveforms to illustrate two different embodiments of the operation of the circuit in FIG. 6a.

[0028] For simplicity in description, identical components are labeled by the same numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Referring to FIG. 2a and 2 b, it is observed that in both IAPT (FIG. 2a) and APT (FIG. 2b) driving schemes, during the period of one field, such as field 2×N or field 2×N+1 in FIGS. 2a, 2 b (N being a positive integer), the SEG/column electrode voltage swings have a very limited dynamic range as compared to the COM/row electrode voltage swings. This invention involves the use of separate power supply systems to provide narrow range, voltage power supply for SEG/column drivers, and wide range voltage power supply for the COM/row drivers.

[0030] Floating V_(COM) Embodiments

[0031] In an embodiment of the invention for obtaining the signals illustrated in FIG. 2b, one floating high voltage V_(COM) supply or two fixed high voltage V_(COM) supplies can be used for the COM electrodes (discussed below). Either way, due to the fact that the current used to supply power the column electrodes are driven through a very limited dynamic range, this causes the power consumption of such a modified system to be lower than the conventional one-power supply driving systems by as much as 80% (see calculation above).

[0032] Floating V_(BIAS) Embodiments

[0033] In one embodiment of the invention for obtaining the signals illustrated in FIG. 2a, one floating low voltage supply system 12 in FIG. 3a provides electrical potentials V_(B+)/V₀/V_(B−) that are used to supply the power for SEG/column switching. This V_(BIAS) supply 12, by proper control of switches, will be switching between three states:

[0034] Charge: where a power source, such as a charge pump or an buffer amplifier, is used to charge up capacitors connected between V_(B+)/V₀/V_(B−) periodically. The capacitors then provide two “operating voltages” for driving the column electrodes.

[0035] V-Low: where the V_(B−) is connected to V1 and V₀/V_(B+) will provide V2, and V3.

[0036] V-High: where the V_(B+) is connected V6 and V₀/V_(B−) will provide V5, and V4.

[0037] As will be discussed in detail in the following paragraphs, a floating V_(COM) structure may be simpler than a floating V_(BIAS) structure. In addition, there will be some minor power consumption for switching V_(BIAS) between the three different states. However, a practical implementation of floating V_(COM) structure in a single chip CMOS IC implementation may call for the use of triple well CMOS process, while the floating V_(BIAS) structure can be achieved in the more conventional twin well CMOS process. Therefore the former approach (i.e. floating V_(COM)) is more suitable for larger LCDs where a multiple-chip solution is required to handle the driving loads, while the latter approach (i.e. floating V_(BIAS))is more suitable for smaller LCD where single chip solution is more economical.

[0038] Either of these two schemes will achieve very significant power savings compared to conventional single power supply system by utilizing the power more efficiently.

[0039] Floating V_(COM) Embodiments

[0040] Using Two Fixed V_(COM) Supplies

[0041] One of the embodiments of the invention involves the use of non-folding, APT driving scheme, (FIG. 2b) and utilizes two fixed V_(COM) supplies as illustrated in FIG. 3a and FIG. 3b. As will be understood by those in the art, while the driver circuit for driving only one row or column electrode is shown in the description below in reference to FIGS. 3a-3 d, 4 a-4 c and 5 a, 5 b, similar circuits may be employed for driving other row and column electrodes. While all of the column electrodes need to be driven at the same time, in some embodiments, only one row electrode is scanned at any one time, so that by means of a switching circuit, only one driver may be adequate for driving sequentially all of the row electrodes.

[0042] In FIG. 3a, the COM or row electrode is scanned by application of either V0<=>V_(COM+) pulses or V0<=>V_(COM−) pulses, depending on the polarity of the row being scanned. These pulses are supplied to the connection node COM of each row electrode by applying appropriate timing signals S+, S0, S− to control the switches I0, I1, I2. The driving potential V_(B+) or V_(B−) is applied to a connection node SEG for each column electrode by applying suitable timing signals SS+(j), SS−(j) to switches I3, I4. The notations “SS+(j), SS−(j)” indicate timing signals suitable for the jth column electrode, where j ranges from 1 to the maximum number of column electrodes. The power consumption of the system is very efficient as compared to conventional single power supply IAPT driving system. As the power consumed by the SEG/column electrodes will be supplied by a dedicated V_(BIAS) low voltage power supply, therefore the current loading of the SEG/column electrodes will have a lower V multiplier factor and therefore the system power requirements are reduced.

[0043] However, in this embodiment, V_(COM+), V_(COM−) are permanently connected to the rest of the driver circuit and therefore require the switch circuit I0 and I2 to tolerate a breakdown voltage of |V_(COM+)−V_(COM−)|. Since |V_(COM+)−V0|=|V0−V_(COM−)|=V_(COM) and can be as high as 15˜18V, the total breakdown requirement can be as much as 30˜36V, which is quite expensive to achieve in fine geometry CMOS process. An improvement in this respect is therefore desirable.

[0044]FIG. 3b is a schematic view of a circuit diagram illustrating a portion of a power supply for generating electrical potentials suitable for driving a LCD. The portion of the power supply shown in FIG. 3b is for generating a suitable voltage for driving the row electrodes; the portion of the circuit used for generating electrical potentials for driving the column electrodes is similar to that of circuit block 12 of FIG. 3a. To simplify FIG. 3b, such circuit block is omitted, it being understood that such circuit block is connected to node V0 of FIG. 3b in the same manner as shown in FIG. 3a. The same is true for FIG. 3c. In FIG. 3b, an improved scheme is illustrated. In this scheme, there are still two fixed high voltage supplies, V_(COM+) and V_(COM−). However, two pairs of switches (I1/I5 and I4/I3) are used to select the power supply for all the row/COM electrode driving circuits. With these switches and the control signal F1 a, F2 a(where F1 a is basically the inverse of F2 a), according to APT driving technique, at any single time, only V_(COM+), or only V_(COM−) is applied, but never both. As a consequence, the voltage across switches I0 and I2 will be limited to |V_(COM)| rather than 2×|V_(COM)|. For example in field 2N, only V0 and V_(COM−) are applied. The break down requirement is therefore reduced by two as compared to FIG. 3a embodiment. In addition, all four power supply switches (I5, I6, I7, I8) only went through one |V_(COM)| transition, and therefore is also required to tolerate one V_(COM) for break-down tolerance.

[0045] The embodiment of FIG. 3b still requires two separate power supplies V_(COM+) and V_(COM−). A further enhancement to FIG. 3b is show in FIG. 3c. In this embodiment, only one voltage supply V_(COM+) is used. In the positive cycle, signal F1 a is High and switches I9, I11 are ON. The V+ connection node will connect to V_(COM+) and V− will connect to V0, while capacitor C0 will be charged to V_(COM+). In the negative cycle, F2 a will be high, switch I10 will be ON, and the electrical potentials from COM/row scanning system 20 is supplied by capacitor C0, which was charged to V_(COM+) during the positive cycle. With I10 turned ON, C0's top terminal is connected to V0, and therefore push C0's bottom terminal to −V_(COM+), or V_(COM−). As a result, by connecting different terminals of C0 to power supply V_(COM+) and to reference node V0, and using the other terminal as V_(COM+) or V_(COM−), only one voltage source is required for the scanning of all row/COM electrode. This is possible because at any single moment in time, there will be only one row/COM electrode scanned in the APT driving scheme. This one single voltage source can supply the voltage required for both the polarity of row/COM scanning pulses.

[0046] This technique can be applied equally well to field inversion APT driving scheme (where the polarity will stay the same for all rows/COMs within one field, and change to another polarity for all rows/COMs for the next field), for row inversion APT driving schemes (where the polarity for one field will be +−+−+− for one field, and −+−+−+ for the next field, etc.) or other inversion driving schemes. Another important advantage of this single V_(COM) scheme is the natural ability to provide balanced voltage supply to both of the polarity of driving signals. This balance is important as any significant DC offset applied across LCD pixels can cause ionization of the LC (liquid crystal) material and damage the applications where the displayed image is essentially static and field to field image change is very limited. Another way of looking at the current reuse is to see the voltage being halved at each of the above transition (flowing first from V_(B+) to V0 and then from V0 to V_(B−) and the V0 will stay essentially unchanged, assuming the load current produce negligible DC drift on V0). In either perspective, the power consumption is cut in half by the C_(B+)/C_(B−) capacitor connected to V0.

[0047] Yet another aspect of this invention is related to the two capacitors C_(B+)/C_(B−) connected between V_(B+) to V0 and V0 to V_(B−). The importance of C_(B−) in terms of current reuse has been discussed above. The significance of C_(B+) can be explained as follows. In order for the above current reuse scheme to work, the V0 node can not be “driven” within two consecutive filed (in other word, V0 can only be driven fairly infrequently, with an average frequency much lower than the field rate, ideally under 15˜20 Hz, or about 20% of the field rate) and certain small percentage of V0 drifting (1˜2%, +/− balanced) has to be allowed. This low frequency driving requirement is intrinsically conflicting with the requirement of V0 being the mid-point between V_(B+) and V_(B−), which comes from the requirement of no net DC voltage being applied across pixel electrodes, as this will require V0 being controlled at all time.

[0048] The addition of C_(B+), which is of substantially the same capacitance as C_(B−), will therefore be important for the accomplishment of the current reuse scheme, as a capacitor voltage divider will be formed by C_(B−) and C_(B+), and any fluctuation occurring across V_(B+) to V_(B−), will automatically be reflected to V0 via the capacitor voltage divider, and therefore help V0 maintain its potential at (V_(B+)−V_(B−))/2. Without C_(B+), the fluctuation of V_(B+) will be applied to only one side of V0 and a net DC voltage can easily develop across LCD pixels. Other than using an extremely precise voltage source for V_(B+), which will be costly, the other way to overcome this DC problem is to use a fairly strong driver to fixate V0 at (V_(B−)−V_(B+))/2. Since this driver can not differentiate between unwanted drifting and the “good drifting” caused by currents to be reused, it may diminish any power savings, and render the current reuse scheme inoperable.

[0049] Floating VBIAS Embodiments

[0050] The above embodiments will achieve the desirable power saving by utilizing a separate power supply for SEG/column electrodes with the low voltage V_(BIAS) . However, due to the dual polarity nature of the circuit structure, the CMOS implementation of these schemes in a single chip LCD controller IC still requires the use of triple-well CMOS process (N-Well for P-MOS, when P-MOS operates in positive voltage, and P-Well for N-MOS, when N-MOS operates in the negative voltage). Triple-Well CMOS process can be ˜20% more expensive as compared to a otherwise equal twin-Well CMOS process (equal in terms of break down voltage, minimum geometry, layers of conductors, etc.). It is therefore desirable to modify the scheme to allow the more economical process to be used.

[0051] Referring the field-inversion IAPT driving scheme in FIG. 2a, it is observed that, during each field, the voltage swing of SEG/column is limited to a relatively smallvalue, on both sides of (i.e. above and below) the non-scanning voltage (either V5 or V2, depends on the polarity of scanning COM/row pulses). It is therefore desirable to provide a “floating” V_(BIAS) power supply system for the column/SEG switching needs (referring to FIG. 4a during each field in such a way that this “floating” power supply will substantially maintain its source voltage (V_(B+)/V_(B0)/V_(B−), or V_(BIAS) for short) stability and supply the current necessary to support the electrode swing between V6-V4 during even fields and V1-V3 during odd fields. With this “floating” V_(BIAS) power supply, then the IAPT folded driving scheme can be used and this allows a complete LCD driving system to be build using a single polarity LCD driving system to be built using twin-well CMOS process.

[0052] A preferred embodiment of this “floating” V_(BIAS) supply system 100 comprises a dedicated fixed high voltage V_(COM) power supply, a V_(BIAS) power supply and a switching circuit for generating certain appropriate electrical potentials for driving the COM or row and SEG or column electrodes. System 100 is shown in FIG. 4a, where node V6 is connected to a dedicated fixed high voltage V_(COM) power supply supplying a voltage V6, and nodes V3/V2/V1 are connected to a V_(BIAS) power supply to supply the three voltages V3/V2/V1. The voltage level for V1 is typically GND and V3−V2=V2−V1=V_(BIAS). Control signal F2 b is the inverse of F1 b (FIG. 4b), with a tiny gap between the ON portions of these two signals to guarantee that no simultaneous turn ON of both power paths is possible.

[0053] In the odd fields, F2 b is “1”, switches I22, I24, I26 are ON, and capacitors C_(B+)/C_(B−) are connected to V3/V2/V1, and system 100 will supply the voltage source to produce SEG/column driving signals between V1

V3, and COM/row scanning pulses between V2

V6.

[0054] In the even fields, F1 b is “1”, capacitors C_(B+)/C_(B−) are disconnected from V3/V2/V1, and switch I23 connects the top terminal C_(B+)t of C_(B+) to V6, which pulls up the other two terminals V_(bo), C_(B−)b of capacitors C_(B+)/C_(B−), assuming C_(B+)/C_(B−) capacitance is substantially higher than the loading current on nodes V5 and V4 generated by switching activities of SEG/column drivers I31/I32, then C_(B+)/C_(B−) will function as a steady voltage source for V5 and LCD permanently. By using only one supply voltage for V_(COM), this balance is automatically established; otherwise, this may require additional high breakdown voltage feedback control circuits and may be difficult and costly to accomplish.

[0055] Yet another improvement to FIG. 3c is illustrated in FIG. 3d. In this embodiment, the connection to V0 is replaced by connection to V_(B+) and V_(B−). In addition V_(B−) is connected to GND while V_(B+) is connected to one single power supply V_(B2). This improved scheme further reduces the complexity of power supply for V_(B+), V_(B−). In addition, the connection of V_(COM) power supply to V_(B+) and V_(B−) allows the current used in supplying V_(COM) to flow through the storage capacitor for V_(B+) (and V_(B−)) and the capacitor formed by the row and column electrodes COMi, SEGj (shown in dotted lines in FIG. 3d) that are addressed at a particular pixel and be reused by the column/SEG electrode drivers for subsequent addressing. A further advantage of moving the V_(COM) connection away from V0 is that V0 serves the important role of establishing balance between V_(B+) and V_(B−) in order to minimize DC voltage as seen across pixel electrodes. If the loading of V_(COM) is through node V0, then all the row/COM scanning current will be loaded on V0, which make balancing more difficult to achieve and a strong voltage follower may be desirable to drive V0. When the V_(COM) is connected to V_(B+) and V_(B−), V0 loading will come completely from column/SEG via LCD pixel capacitance coupling (e.g. COMi, SEGj in FIG. 3d), which is, by definition of capacitor coupling, DC free, and a device driving node V0 can be very weak (just enough to handle C_(B0) leakage and the unbalanced part of the coupled signals).

[0056] Current Reuse

[0057] It will be noted that in FIG. 3b, 3 c or 3 d, the low voltage supply system V_(BIAS) involves the use of capacitor C_(B+) and C_(B−). When a SEG electrodes swing from V_(B+) to V_(B−), or vise versa, a current is coupled to the node V0 via the LCD panel capacitance. This current will be a “source” current (flowing out from V0 node) for V_(B+) to V_(B−) transition and “sink” current (flowing into V0 node) for V_(B−) to V_(B+) transition. As just discussed above, these currents are intrinsically balanced, and therefore result in no DC in the long term. Now, if a sufficiently large capacitor (for example over 150× the total LCD panel load capacitance) is connected between V0 and ground, then another very important characteristics will emerge, which is current reuse: the current used by one row of pixels for transitions from, say, V_(B+) to V_(B−) in one field will supply the same row's transition from V_(B−) to V_(B+) in the next field. Essentially, the current pulses will be used twice in two consecutive fields, but temporarily stored in the capacitor connected to V0. This is especially true for almost all passive LCD display V4 through switch I21 and I25. With V6, V5, V4, the system 100 driving SEG/column electrodes (only one pair of drivers is shown for one channel) will produce driving signals between V6

V4, while the system 100 will produce V5

V1 scanning pulses.

[0058] During the even field, since the SEG/column driving power comes entirely from capacitors C_(B+)/C_(B−), for the above system to operate properly under all display patterns, the capacitance of C_(B+)/C_(B−) needs to be substantially (20×˜50×) larger than (the sum of the pixel capacitance)×(maximum possible SEG/column electrode transitions). For a 100×200 pixel LCD panel assuming each pixel has a cell capacitance of 2pF, then the above calculation will imply the capacitance of C_(B+)/C_(B−) needs to be

(20×˜50×)×2 pF×100×200×50=40˜100 uF,

[0059] where 50=100/2 is the maximum possible SEG/column transition, happen when the pattern is black/white interleaved lines or checker board.

[0060] This capacitance requirement is very high and capacitors with such high capacitance values are generally high leakage, electrolytic type capacitors, and therefore are undesirable for ultra-low power battery operated devices.

[0061] An improvement based on the same circuit as the embodiment of FIG. 4a is to modify the power source management signal F1 b/F2 b, and insert “recharge pulses” in F2 pulses during the even field such that C_(B+)/C_(B−) will be recharged periodically (FIG. 4c). For example, by inserting such recharge pulses per row scanning period, the calculation for the capacitor required will reduce to: (20×˜50×)×2 pF×100×200×50/100 =0.4˜1 uF. This range of capacitance can be provided by ceramic capacitor which has negligible leakage current and are in very compact SMD (surface mount device) packages.

[0062] Another important factor for this scheme to function is the utilization of the intrinsic capacitances of LCD panels. During the brief periods where the recharging pulses are asserted during the even field, the intrinsic capacitance of the LCD performs as the holding capacitor for the SEG/column signal until capacitor C_(B+)/C_(B−) is reconnected to produce V5/V4. To achieve this, the output of SEG/column driver portion is temporarily turned off by setting both SS+ and SS− to “0”, and therefore render the SEG/column drivers at a high impedance state to preserve the charge stored in the intrinsic capacitance of the LCD.

[0063] With the above structure, all control signals and switches will operate in the positive (or negative) voltage range (relative to GND). This is particularly advantageous where the structure is implemented as an integrated circuit, since the operating voltages can be all positive or negative in reference to the substrate potential. Therefore, other than the capacitors C_(B+)/C_(B−), all control signals and switches can be implemented by a conventional twin-Well CMOS process.

[0064] Practically all varieties of commercial passive LCD driving systems involve an orthogonal driving waveform which generally comprise a low voltage swing SEG/column driving system, and a high voltage pulses COM/row scanning system. The relative magnitude between SEG/column and COM/row scanning is generally decided by {square root}{square root over (Mux)}, where Mux is the multiplexing rate. For single row scanning system such as APT or IAPT driving schemes, the Mux rate equals the number of rows of the LCD. In MLA (multiple line addressing) driving schemes, Mux=Row/L, where L is the number of rows driven simultaneously at each row scanning period.

[0065] Because of significant difference of voltage levels between these two voltage requirements, it is usually undesirable to share one voltage supply between the SEG/column drivers and COM/row drivers. This invention introduces several embodiments for driving passive LCDs using two sets of power supplies to reduce the total power consumption of the LCD display system (including the power consumed by the LCD panel and by the driver). In addition, this invention describes several suitable circuit structures and control signal techniques for CMOS implementation to allow effective IC implementation of the introduced concept/method.

[0066] Although the above discussion is limited in its scope, the application of the power system structure system can be expanded to other types of passive LCD driving systems such as pulse-width modulation of SEG/column signals, frame-rate corrected gray-shade modulation and various MLA or active addressing methods. In each of the derived applications, the voltage levels need to be adjusted to optimally conserve the power, and the interaction with driving signals (especially SEG/column) needs to be properly managed to minimize the impact of the image quality.

[0067] In the embodiments of FIGS. 3a-3 d, a first power supply is used to supply electrical potentials dedicated for driving row or COM electrodes, and a second power supply separate from the first is used to generate electrical potentials dedicated and suitable for driving a column or SEG electrodes. In the embodiment of FIGS. 4a-4 c, however, it will be noted that the power sources V6, V3, V2 and V1 are used in combination for generating electrical potentials suitable for driving both row and column electrodes. Thus as described above, capacitors C_(B+)/C_(B−) in combination with the voltage sources V6, V3, V2 and V1 are used to generate electrical potential V5 for driving the row electrodes as well as electrical potential V4 for driving the column electrodes.

[0068] It will be noted that in the description herein concerning embodiments employing capacitors, a power supply supplying a particular voltage is described as charging the capacitor to the same voltage. In practical circuits, however, it is understood that a capacitor is charged in a practical time frame to a desired voltage which is typically lower than the voltage supplied by the power supply. For example, if the two capacitors C_(B+) and C_(B−) are to be charged to 1.2 volts across each of the two capacitors, the voltage supply that is used to charge the two capacitors may need to supply a voltage of about 3.0 volts, rather than 2.4 volts in a practical implementation. It will be understood that, in all of the embodiments of the invention described herein involving the charging of capacitors, even though the description of such embodiments may indicate that the capacitors are charged to the same voltage as that supplied by the power supply, it will be understood that in actual practice, the two voltages are only substantially the same, preferably, with the voltage supplied by the power supply slightly higher than that of the desired voltage to which the capacitor is to be charged.

[0069]FIGS. 5a and 5 b together are a schematic circuit diagram of a power supply circuit for driving the LCD to illustrate the preferred embodiment of the invention. As shown in FIGS. 5a and 5 b, the overall circuit of the power supply system may be naturally divided into two portions: a first portion 200 a for generating electrical potentials suitable for driving row and/or column electrodes and a second portion 200 b for applying the electrical potentials supplied or generated by portion 200 a to the row and column electrodes at the appropriate times. The first portion 200 a is shown in FIG. 5a. As shown in FIG. 5a, three voltages are provided by two or more power sources: V6, VC and V1, where V1 may simply be ground potential. Two capacitors C1 and C2 are employed for storing energy during a first phase of the operation of combined circuit 200 a, 200 b. During the second phase, the energy stored in the capacitors is then used, together with the power sources V6, V1, for generating electrical potentials suitable for driving row and/or column electrodes.

[0070] The voltage VC is chosen so that the voltage difference between VC and V1 is substantially equal to the amplitude of the voltage difference between non-scanning row electrodes and column electrodes. Thus, during the first phase of operation of the combined circuit 200 a, 200 b, the control signal F3 c is applied to switches I34, I36, I38 and I40, turning on these switches, so that capacitors C1 and C2 are connected in parallel to the voltage VC and V1. Terminals C1 t and C2 t of C1 and C2 are thus charged to voltage VC and the remaining terminals C1 b and C2 b of the two capacitors are at the reference voltage V1.

[0071] During the second phase of the operation of the combined circuit 200 a, 200 b, control signal F3 c is pulled low thereby turning off switches I34-I40. Then the control signal F1 c or F2 c is pulled high. Switches I44, I48 are of the type that they will turn on whenever either F1 c or F2 c is high. For convenience in description, it is assumed that F1 c is pulled high first and not F2 c. Then switches I42, I44, I48, I50, I56, I60, I64 and I68 are turned on. This causes terminal C1 t of capacitor of C1 to be pulled to voltage V6. Since capacitor C1 has previously been charged during phase 1 to a voltage difference of (VC−V1) between its terminals, then the other terminal C1 b of capacitor C1 is thereby also pulled to a value V5 so that the voltage difference (V6−V5) equals (VC−V1). The voltage V5, however, will be floating since it is not connected to any one of the three voltage supply nodes V6, VC and V1. Node VM will therefore be at voltage V5 through open switch I44. Terminal C2 t of capacitor C2 is also connected to node VM so that it is pulled to voltage V5 and its other terminal C2 b is pulled to a value V4 so that the voltage difference (V5−V4) is equal to the voltage difference (VC−V1). This is the case since the capacitor C2 has been charged during the previous phase 1 to this voltage difference. Thus, the voltage V5 passes through switch I44 and appears at node VM and the voltage V4 passes through switch I48 and appears at node 202.

[0072] In reference to FIG. 5b, the voltage V5 appearing in node VM passes through switch I56 and appears at node VC+ and reference voltage V1 passes through switch I60 to appear at node VC−. Therefore, by controlling switches I0 and I2 by means of control signals S+ and S− as before, the voltages V5 and V1 are applied to one or more COM or row electrodes. The voltage V6 also passes through switch I64 and appears at node VS+ and the voltage V4 at node 202 passes switch I68 to appear at node VS−. As before, by applying suitable timing control signals SS+(j) and SS−(j) to control switches I32 and I31, appropriate pulses of V6<=>V4 are applied to the corresponding column electrode SEGj. In reference to FIG. 2a, the above described driving voltage waveforms are the ones suitable for field 2×N, where the electrical potentials applied to the COM electrodes are between V5 and V1 and those applied to the column electrodes are between V6 and V4. As noted above, the voltages V5 and V4 are not connected to any power supply during this time and, therefore, float in reference to voltage V6 through the capacitors C1 and C2.

[0073] During the second phase, during another field addressing cycle, such as field 2×N+1 in FIG. 2a, the control signal F3 c remains low, control signal F1 c is pulled low and control signal F2 c is pulled high. This turns off switches I42, I48, I56, I60, I64 and I68, and turns on switches I46, I52, I54, I58, I62 and I66. Switches I44 and I50 remain on whenever either one of the control signals F1 c and F2 c is high. Terminal C2 b of capacitor C2 is pulled to V1 through switch I52. Since the capacitor C2 has previously been charged during the first phase to (VC−V1), the terminal C2 t will be pulled to a voltage V2 where (V2−V1) is equal to (VC−V1), where V2 floats in relation to V1 through capacitor C2. This voltage V2 passes switch I50 and appears at node VM. Terminal C1 b of capacitor C1 is connected to VM through open switch I44 so that terminal C1 b is also at voltage V2. Since the capacitor C1 has previously been charged to a value (VC−V1) during the first phase, the terminal C1 t of capacitor C1 will be pulled to a value V3, where (V3−V2) is equal to (VC−V1), where V3, therefore, also floats with V2 and V1 through capacitors C1 and C2.

[0074] The voltage V3 passes through switch I46 and appears at node 204. In reference to FIG. 5b, voltage V2 appearing at node VM passes through switch I58 and appears at node VC− through switch I58 and the voltage V6 passes through switch I54 to appear at node VC+. Therefore, by means of timing control signals S+ and S−, the voltages V6 and V2 are applied through switches I0 and I2 to a selected COM electrode. Reference voltage V1 passes through switch I66 and appears at node VS− and the voltage V3 passes switch I62 to appear at node VS+. As before, by means of timing control signals SS+(j) and SS−(j) controlling switches I32 and I31, respectively, the voltages V3 and V1 are applied to the corresponding electrode SEGj, where j ranges from 1 to the maximum number of column electrodes, such as k in FIG. 1. The operation of the combined circuit 200 a, 200 b of FIGS. 5a, 5 b during the two phases is summarized in the table below, where the underlined voltages are the ones that are floating: F3c F1c F2c C1t VC V6 V3 C1b V1 V5 (V_(c) − V₁ = V₆ − V₅) V2 (V_(c) − V₁ = V₃ − V₂) C2t VC V5 V2 (V_(c) − V₁ = V₂ − V₁) C2b V1 V4 (V_(c) − V₁ = V₅ − V₄) V1

[0075] With the above structure in FIGS. 5a, 5 b, all control signals and switches will operate in the positive (or negative) voltage range (relative to GND). This is particularly advantageous where the structure is implemented as a single-chip integrated CMOS circuit, since the operating voltages can be all positive or negative in reference to the substrate potential. Therefore, other than the capacitors C1 and C2, all control signals and switches can be implemented by a conventional twin-Well CMOS process.

[0076] In the embodiment of FIGS. 5a, 5 b, the operation of the circuit has two distinct phases: a first phase during which one or more capacitors is charged and during which no electrical potentials are applied to the row or column electrodes. This is followed by phase 2 where one of the two terminals of each capacitor is connected to a voltage supply so that the electrical potential of the remaining terminal of the capacitor is pulled to a value equal to the voltage difference (VC−V1) from that of the voltage supply. One or more of the voltage supplies V6 and V1 from the power supplies and one or more of the generated floating voltages at such other terminal of each of the two capacitors are then used to control and drive the row and column electrodes.

[0077] In other embodiments described above, such as those in FIGS. 3c, 3 d and 4 a, there are also two phases where during the first phase, one or more capacitors is charged, and where during the second phase, a floating voltage at a terminal of the capacitor(s) and one or more supply voltages are used for driving row and/or column electrodes. In such embodiments, however, the supply voltage or voltages are also used during the first phase to drive the row and/or the column electrodes. All such variations are within the scope of the invention.

[0078] In the embodiment of FIG. 4a, it may be important to employ capacitors C_(B+) and C_(B−) that are of substantially equal value and employ voltage supplies V3, V2, V1 so that (V3−V2) is substantially the same as (V2−V1). This is the case to maintain a zero voltage offset at the column electrodes. In the embodiment of FIGS. 5a, 5 b, this is assured since the capacitors C1 and C2 are charged in parallel during the first phase to the same voltage difference. This reduces the tolerances for components that may be used in the drivers so that lower cost components may be used. In the embodiment of FIG. 4a, the two capacitors are charged when they are placed in series whereas in the embodiment of FIGS. 5a, 5 b, the two capacitors C1 and C2 are charged when there placed in parallel between the same two voltage sources.

[0079] Thus, the feature common to the embodiments in FIGS. 3c, 3 d, 4 a, 5 a and 5 b is that two or more power supplies together with a circuit including an energy storage device are employed to generate electrical potentials for driving row and column electrodes. The circuit causes at least one of the electrical potentials so generated to float with a voltage supplied by one of the power supplies. Thus, in the embodiment of FIGS. 3c, 3 d, when the reference voltage V0 is connected to node V+, thereby putting one terminal of the capacitor C0 to the reference voltage, the other terminal of capacitor C0 is pulled to the floating value—V_(COM+) or V_(COM−.)

[0080] In the embodiment of FIG. 4a, when the capacitors C_(B+) and C_(B−) are pulled up by voltage supply V6, this causes the node between the two capacitors and the remaining terminal of capacitor C_(B−) to also be pulled to floating values useful for driving the row or column electrodes. Thus, the voltage value at the node between the two capacitors is pulled to a value V5 where (V6−V5) is substantially equal to (V3−V2) which is the voltage difference by which the capacitor C_(B+) has been charged during the previous phase. At the same time, the voltage at node VS− will also be pulled up to the voltage at the bottom terminal C_(B−t) which is at a voltage V4, where (V5−V4) is substantially the same as (V2−V1). This is the case since capacitor C_(B−) has previously been charged during the previous phase to (V2−V1).

[0081] Yet another feature common to the embodiments of the invention in FIGS. 3c, 3 d, 4 a, 5 a and 5 b is that the driver circuit as a whole experiences a voltage dynamic range which does not substantially exceed the amplitude of the voltage pulses applied to the row electrodes. In reference to FIG. 2b, for example, where two separate voltage sources are employed to generated the positive and negative going voltage pulses for application to row electrodes, the power supply circuit would experience a voltage dynamic range equal to the sum of the amplitudes of the positive and negative going pulses. Since the positive and negative going pulses should have equal amplitudes for zero DC offset, this means that such power circuit would experience twice the amplitude of the voltage pulses applied to the row electrodes. In the example shown in FIG. 2b, the dynamic range experienced by the power circuit would be equal to V_(COM+), which is equal to V_(COM −). In the embodiments of FIGS. 3c, 3 d, 4 a, 5 a and 5 b, since capacitors have been used to enable a single power supply to generate both the positive and negative going voltage pulses for row electrodes, the voltage dynamic range seen or experienced by the power supply circuit as a whole is substantially equal or not significantly higher than the amplitude of the positive and negative going pulses applied to the row electrodes. In other words, such feature enables the total voltage dynamic range to be reduced as in the case of IAPT, while at the same time vastly reducing the power consumption by the driver when compared to IAPT.

[0082] From the description above, it will be evident since the column (SEG) electrodes are driven either directly by a reference voltage (from a power source or supply, or connected to ground or other reference source) or directly by an energy storage device such as a capacitor (or inductor), so that the above-described problems associated with Class B bias circuits are avoided altogether. It is due to the fact that the driver circuits described above do not employ N and P transistors that are turned off at low error voltage for supplying energy to the column electrodes. Thus, assuming that the energy storage sources are able to drive the column electrodes to substantially their target values within each row scanning cycle, the above described crosstalk problem exhibited by conventional LCDs is avoided or alleviated.

[0083] Where energy storage devices such as capacitors are used to supply current to column electrodes, in order to drive the column electrodes to target voltage values, the accuracy at which these target values can be accomplished depends on the capacitance of the driving capacitor relative to the load capacitance of the column electrode that is driven by means of the capacitor. Thus, if the capacitance of the driving capacitor is 20 times that of the load capacitance at the column electrode being driven, the capacitor can drive the column electrode to within 5 percent of the target voltage value. Thus, in order to further increase driving accuracy so that the error is less than 5%, even larger capacitors may need to be used. Another aspect of the invention is based on the recognition that, by using two or more sets of capacitors to drive the column electrode during the same row-scanning period, the above requirement of using large value capacitors for achieving accurate target values may be relaxed. According to this aspect of the invention, a column electrode may be driven by a first set of one or more capacitors to a voltage value which is close to but not at the target value, and then a second set of one or more capacitors is then used to drive the column electrode to substantially the target value. Using such driving scheme, the capacitances of the capacitors in the first and second sets need not be huge. For example, if the capacitances of capacitors in the first and second sets are each 20 times or more than the load capacitance, then during the first phase, the column electrode is driven to within 5 percent of the target value. During the second phase of the same row-scanning cycle, the second set of capacitors is then used to drive the column electrode to within 0.25 percent of the target value, or substantially at the target value.

[0084] This aspect of the invention is illustrated in FIGS. 6a, 6 b. As shown in FIG. 6a, circuit 300 includes a power supply 302 which includes a current source 304, a comparator 306 and a reference voltage 308. Voltage reference source 308 provides a reference voltage V_(REF) relative to the ground. Circuit 300 includes two sets of capacitors: a first set comprising capacitors C1, C2 and a second set comprising capacitors C3, C4. The two sets of capacitors are used alternately to supply voltages or electrical potentials to three nodes: N1, N2 and N3. The voltages or electrical potentials at these three nodes are then used for driving each of the column electrodes (SEG) to one of a set of three different potentials as required: V1, V2 and V3. The same scheme may be used for achieving the set of potentials V4, V5 and V6 as well.

[0085] When the first set of capacitors is being used to supply current to the three nodes, the second set of capacitors is connected to the power source or supply 302 for charging the second set of capacitors. This occurs during one cycle of the switching circuit 300. During the next cycle, the second set of capacitors is then used to supply power to the three nodes while the first set of capacitors is then charged by the power supply 302. The operation of the switches and the alternate operation of the two sets of capacitors are illustrated in reference to the timing diagram of FIG. 6b.

[0086] Circuit 300 may be operated in two different operations illustrating two different embodiments. The first embodiment is illustrated by the timing diagram 320 labeled S1 through S4, which are graphical illustrations of the On-Off states of the switches S1-S4 of circuit 300. The second set of timing diagrams 330 and labeled S1′, S2, S3′ and S4 illustrate a second embodiment in operating circuit 300.

[0087] Each of the two dotted lines t0 and t0′ marks the starting point in time of the scanning of a new line or row electrode. Thus the time period t0-t0′ defines a row-scanning period or a row-scanning cycle. During a row-scanning cycle, it is desirable for power supplied by circuit 300 to drive each of the column electrodes to the target voltage or electrical potential as quickly as possible. During the row-scanning cycle t0-t0′, the switches S1 and S4 are turned off during a greater part of the cycle and switches S2 and S3 are turned on. Thus, during a greater part of the cycle, the second set of capacitors C3, C4 are connected to nodes N1, N2, N3 and disconnected from the power supply 302. For example, node C3 t is connected to node N1 and node C3 b is connected to node N2. Node C4 t is connected to node N2 and node C4 b is connected to node N3. Therefore, the potential difference or voltage across nodes N1, N2 is substantially equal to the voltage across capacitor C1, and the voltage across nodes N2, N3 is substantially equal to the voltage across capacitor C2. Since switches S4 are off most of the time during this row-scanning cycle t0-t0′, capacitors C3, C4 are not connected to the power supply 302 during a greater portion of this row-scanning cycle.

[0088] In the same vein, since switches S4 are turned off and switches S3 are turned on during a greater part of the cycle, the first set of switches is disconnected from the nodes N1-N3 and connected to the power supply in order to replenish the charges in the capacitors that were drained during the previous row-scanning cycle. In this manner, during the row-scanning cycle immediately following time t0′, the first set of capacitors would be fully charged and then ready to supply energy to nodes N1, N2, N3.

[0089] While it is true that during a major portion of the row-scanning cycle t0, t0′ the second set of switches are connected to nodes N1-N3 and the first set of switches are connected to the power supply 302, it will be observed from FIG. 6b that the switching cycle S1-S4 are delayed by an offset t0-t1 relative to the starting time t0 of the row-scanning cycle. This is preferable to achieve a greater accuracy for driving column electrodes connected to nodes N1, N2, N3 to the desired target voltage values. As shown in FIG. 6b, in order to drive a column electrode from voltage V1 to voltage V3, for example, the first set of capacitors C1, C2 is still connected to nodes N1, N2, N3 at time t0. In fact, as will be explained below, during this short time period t0-t1 that makes up only a small portion of the row-scanning cycle t0-t0′, a major portion of the charges originally stored in capacitors C1, C2 is now transferred during this short time period to the column electrode by circuit 300 as shown in curve Emb.1 in FIG. 6b. Thus, at time t1, the voltage of the column electrode connected to circuit 300 is driven from a value of V1 at time t0 to a value V′ which is close to the target voltage V3 at time t1. At time t1, switches S1, S4 are turned off and switches S2, S3 are turned on. This causes capacitors C1, C2 to be disconnected from nodes N1-N3 and capacitors C3, C4 to be connected to the nodes instead. At this time t1, capacitors C3, C4 have been fully charged by the power supply 302 and are therefore very effective in quickly driving the same column electrode that was driven to voltage V′ by C1 and C2 to a value very close to the target voltage V3, as shown in curve Emb.1 in FIG. 6b.

[0090] For example, if the capacitance of capacitors C1, C2 is 20 times that of the load capacitance, this means that V′ will be less than V3 by only about 5 percent or so of V3. If the capacitance of capacitors C3, C4 is also 20 times that of the load capacitance, this means that when the second set of capacitors is used to drive the same column electrode from V′ towards voltage V3, the resulting voltage of the column electrode will be within about 0.25 percent less than V3. In this manner, the column electrode can be driven to a voltage substantially equal to the target voltage value without having to use large value capacitors.

[0091] From the above, since the column electrode is already driven to the voltage V′ by capacitors C1, C2, where V′ is close to the voltage V3, the second set of capacitors C3, C4 will only need to supply a small amount of charge and current in order to drive such column electrode towards the value substantially equal to V3. Therefore, at time t0′, only a minor portion of the charges have been drained from capacitors C3, C4. Therefore, during the subsequent short time period t0′-t1′, a major portion of such charges will then be transferred to the column electrode connected to nodes N1-N3, in order to again drive such column electrode to a voltage value which is close to voltage V3, or some other target voltage.

[0092] In the example illustrated above, the column electrode is driven from voltage V1 to a voltage substantially equal to voltage V3. The same reasoning will apply for the two sets of capacitors to drive any column electrode from any one of the three voltage values V1-V3 to another different voltage value with all the associated advantages explained above.

[0093] Thus, as will be evident from the above, by delaying the timing of the switching cycle 320 relative to the row-scanning cycle by an offset t0-t1, t0′-t1′, . . . , the column electrodes may be driven to values very close to or substantially equal to the target voltage without having to use large-size capacitors.

[0094] The second embodiment 330 differs from the first embodiment 320 in that the starting points of time periods in which the first set of capacitors C1, C2 is turned off is furthered delayed to times t2, t2′, . . . as illustrated by the timing S1′. Thus, during the time period t1-t2, both sets of capacitors C1-C4 will be supplying current and energy to the three nodes, thereby shortening the time which is required to drive a column electrode connected to the nodes to a value substantially equal to V3. The overlapping time period is shown as shaded areas in the waveform 330. The resulting voltage of the column electrode is illustrated by curve Emb.2 in FIG. 6b.

[0095] As will be noted from the switching waveforms 320, 330, the three pairs of switching waveforms S1, S3; S2, S4 and S1′, S3′ are complementary. In this manner, when a set of capacitors is connected to the nodes to supply energy to the column electrodes, this set is disconnected from the power supply; vice versa, when a set of capacitors is connected to the power supply 302 in order to charge the capacitors, this set is disconnected from the nodes.

[0096] Power supply 302 operates as follows. Current source 304 will supply current to one of the two sets of capacitors connected to it until it is switched off by comparator 306. When switches S3 are on, comparator 306 compares the common voltage at node C1 t, C2 t for the first set of capacitors to the voltage reference V_(REF). When the voltage at the common node is raised by the charging action so that it is substantially equal to the reference voltage, comparator 306 turns off the current source 304. During this charging process, capacitors C1, C2 are connected in parallel to the current source 304. Capacitors C3, C4 are charged in a similar manner.

[0097] After the charging process, the voltage across each of the capacitors C1-C4 is substantially equal to the reference voltage V_(REF). Therefore, when the fully charged capacitors are then connected to nodes N1-N3, the voltage across nodes N1, N2 and across nodes N2, N3 would each be substantially equal to the reference voltage V_(REF).

[0098] While two sets of capacitors are employed as illustrated in FIG. 6a, 6 b, it will be understood that a single set may be adequate for some applications, and that more than two sets may be used in other applications; such other variations are within the scope of the invention.

[0099] While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalents. All references referred to herein are incorporated by reference in their entireties. 

What is claimed is:
 1. An apparatus for driving a liquid crystal display, said display comprising an array of elongated row and an array of elongated column electrodes arranged transverse to the row electrodes, wherein overlapping areas of the two arrays of electrodes define pixels of the display when viewed in a viewing direction, said apparatus comprising: at least two separate power sources; and a circuit responsive to the at least two power sources and supplying electrical potentials to the row and column electrodes, to cause the display to display desired images, wherein at least one of the electrical potentials supplied to the row and column electrodes floats with a voltage supplied or caused to be supplied by one of the power sources.
 2. The apparatus of claim 1, the circuit including a control device and at least one energy storage device to supply said at least one of the electrical potentials, wherein the control device causes the at least one energy storage device to be charged in a first phase, and connects said at least one energy storage device to the voltage supplied or caused to be supplied by said one of the power sources and to the row or column electrodes in a later second phase, so that said at least one of the electrical potentials supplied by said at least one energy storage device to the row or column electrodes floats with said voltage.
 3. The apparatus of claim 2, wherein at least one energy storage device has at least two terminals, wherein the control device causes one terminal to be connected to the voltage and another terminal to be connected to the row and column electrodes, wherein said another terminal supplies said at least one of the electrical potentials that floats with the voltage.
 4. The apparatus of claim 2, the at least one energy storage device including one or more capacitors.
 5. The apparatus of claim 2, wherein the control device also causes the at least one energy storage device to supply electrical potentials to the row or column electrodes during the first phase.
 6. The apparatus of claim 2, said at least two power sources supplying respectively a first and a second voltage and a common reference voltage, the difference between the second and the reference voltages defining a voltage differential, said control device comprising a first set of switches that causes a set of voltages to be generated that are above the reference voltage or below the first voltage by an integer multiple of the voltage differential.
 7. The apparatus of claim 6, said control device further comprising a second set of switches that connect said set of voltages at selected times to the row and column electrodes so that the electrodes are driven by an IAPT driving method.
 8. The apparatus of claim 6, wherein some of the voltages in the set of voltages float with the reference voltage during some field addressing cycles and other voltages in the set of voltages float with the first voltage during other field addressing cycles.
 9. The apparatus of claim 2, said circuit including two energy storage devices each having two terminals, wherein the control device causes the two energy storage devices to be connected in parallel to the power sources during the first phase to charge the energy storage devices, so that they are charged to substantially the same voltage across their terminals.
 10. The apparatus of claim 2, said circuit including two energy storage devices each having two terminals, wherein the control device causes the two energy storage devices to be connected in series to the power source during the first phase to charge the energy storage devices.
 11. The apparatus of claim 2 wherein the electrical potentials supplied by the circuit to the row electrodes are of a predetermined amplitude above a reference voltage in some field addressing cycles and of the predetermined amplitude below the reference voltage in other field addressing cycles, wherein the electrical potentials supplied by the circuit have a dynamic range substantially equal to said amplitude.
 12. The apparatus of claim 1, said circuit including two energy storage devices, wherein the devices are charged by one of the power sources during a portion of at least one field addressing cycle and used to supply electrical potentials to a row or column electrode in a different portion of such field addressing cycle, wherein the devices are charged for a fraction of such different portion to compensate for charge consumption.
 13. The apparatus of claim 1, said apparatus being an integrated circuit having a substrate, wherein the first and second power sources supply only electrical potentials that are higher or lower than a reference potential of the substrate.
 14. An apparatus for driving a liquid crystal display, said display comprising an array of elongated row and an array of elongated column electrodes arranged transverse to the row electrodes, wherein overlapping areas of the two arrays of electrodes define pixels of the display when viewed in a viewing direction, said apparatus comprising: at least two separate power sources; wherein one or more of the power sources drives the row electrodes through a first voltage range, and drives the column electrodes through a second voltage range, wherein the first voltage range changes over different field addressing cycles, and the second voltage range floats with the first voltage range when the first voltage range changes and with at least the voltage generated or caused to be generated by one of the power sources.
 15. The apparatus of claim 14, wherein the first voltage range is between a non-scanning voltage value and a scanning voltage value, and wherein the second voltage range floats with the non-scanning voltage value.
 16. The apparatus of claim 15, wherein the first power source includes a first power supply, wherein the second power source includes a pair of capacitors, said apparatus further comprising a switching circuit connecting the first power supply and the capacitors to cause the second voltage range to float about the non-scanning voltage value.
 17. The apparatus of claim 16, wherein the pair of capacitors are connected in a voltage divider configuration separating three nodes, wherein the switching circuit causes one of the nodes in between the pair to be at the non-scanning voltage of the first voltage range in at least one field addressing cycle.
 18. The apparatus of claim 17, wherein the switching circuit causes voltages at one of the two remaining nodes to be supplied to a connections to a column electrode during at least one field addressing cycle.
 19. The apparatus of claim 15, wherein the capacitors are charged by the first or second power source during a portion of at least one field addressing cycle and used to supply electrical potentials to a row or column electrode in a different portion of such field addressing cycle, wherein the capacitors are charged for a fraction of such different portion to compensate for charge consumption.
 20. The apparatus of claim 19, wherein the column electrodes are driven by column drivers, and wherein the column electrodes are substantially disconnected from column drivers during said fraction of such different portion to preserve column signals that have been applied to the column electrodes.
 21. The apparatus of claim 14, said apparatus being an integrated circuit having a substrate, wherein the first and second power sources supply only electrical potentials that are higher or lower than a reference potential of the substrate.
 22. The apparatus of claim 14, one of the power sources including at least one power supply and a capacitor, said capacitor connected to a connection node for each of the column electrodes and storing charges from a column electrode during one column addressing cycle, said second power source further including a switching circuit that cause said stored charges to be applied to said column electrode in a subsequent column addressing cycle.
 23. An apparatus for driving a liquid crystal display, said display comprising an array of elongated row and an array of elongated column electrodes arranged transverse to the row electrodes, wherein overlapping areas of the two arrays of electrodes define pixels of the display when viewed in a viewing direction, said apparatus comprising: at least two separate power sources; and a circuit responsive to the at least two power sources and supplying electrical potentials to the row and column electrodes, to cause the display to display desired images; wherein the electrical potentials supplied by the circuit to the row electrodes are of a predetermined amplitude above a reference voltage in some field addressing cycles and of the predetermined amplitude below the reference voltage in other field addressing cycles, wherein the electrical potentials supplied by the circuit have a dynamic range substantially equal to said amplitude.
 24. The apparatus of claim 23, said at least two power sources supplying a reference voltage and a voltage above or below the reference voltage of an amplitude suitable for driving the row electrodes, said circuit including a capacitor and a selection circuit selectively connecting the at least two power sources and the capacitor to each of the row electrodes, so that positive going pulses of said amplitude in reference to the reference voltage are applied in some field addressing cycles to the row electrodes and negative going pulses of said amplitude in reference to the reference voltage are applied to the row electrodes in other field addressing cycles for the row electrodes.
 25. A method for driving a liquid crystal display, said display comprising an array of elongated row and an array of elongated column electrodes arranged transverse to the row electrodes, wherein overlapping areas of the two arrays of electrodes define pixels of the display when viewed in a viewing direction, said method comprising supplying electrical potentials to the row and column electrodes, to cause the display to display desired images; wherein said supplying includes charging and discharging at least one energy storage device to supply said at least one of the electrical potentials.
 26. The method of claim 25, wherein at least one of the electrical potentials supplied to the row or column electrodes floats with a voltage supplied by a power source, wherein said supplying includes charging the at least one energy storage device in a first phase, and connects said at least one energy storage device to the row or column electrodes in a later second phase, so that said at least one of the electrical potentials supplied by said at least one energy storage device to the row or column electrodes floats with said voltage.
 27. The method of claim 26, wherein said at least one energy storage device has at least two terminals, wherein the supplying causes one terminal to be connected to the voltage and another terminal to be connected to the row and column electrodes, and causes said another terminal to supply said at least one of the electrical potentials that floats with the voltage.
 28. The method of claim 25, wherein said supplying comprises connecting an energy storage device alternately to a power supply and at least one column electrode.
 29. The method of claim 28, said supplying comprising connecting the power supply and the at least one column electrode to a first and a second electrical energy storage device in alternate row scanning cycles.
 30. The method of claim 29, wherein said supplying connects the power supply and the at least one column electrode to the two energy storage devices according to a switching timing waveform that is delayed relative to said row scanning cycles, so that at least one of the energy storage devices is connected to the at least one column electrode during a portion of a row scanning cycle, and the remaining energy storage device is connected to the at least one column electrode during another portion of such row scanning cycle.
 31. The method of claim 30, wherein said time delay is such that a major portion of the energy of the first energy storage device is transferred to the at least one column electrode during a beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to close to a target value, and a minor portion of the energy of the second energy storage device is transferred to the at least one column electrode during but after the beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to substantially the target value.
 32. The method of claim 30, wherein said switching timing waveform is such that during a portion of such row scanning cycle, both the first and the second energy storage devices are connected to the at least one column electrode.
 33. The method of claim 29, wherein the first energy storage drives the voltage of said at least one column electrode to close to a target value during a beginning portion of a row scanning cycle, and the second energy storage device drives the voltage of said at least one column electrode to substantially the target value during but after the beginning portion of such row scanning cycle.
 34. The method of claim 33, wherein a major portion of the energy of the first energy storage device is transferred to the at least one column electrode during a beginning portion of a row scanning cycle, thereby driving the voltage of said at least one column electrode to close to a target value, and a minor portion of the energy of the second energy storage device is transferred to the at least one column electrode during but after the beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to substantially the target value.
 35. The method of claim 29, wherein during a portion of a row scanning cycle, both the first and the second energy storage devices are connected to the at least one column electrode.
 36. An apparatus for driving a liquid crystal display, said display comprising an array of elongated row and an array of elongated column electrodes arranged transverse to the row column electrodes, wherein overlapping areas of the two arrays of column electrodes define pixels of the display when viewed in a viewing direction, said apparatus comprising: a power supply; at least one electrical energy storage device; and a switching circuit connecting the power supply to the device to charge the device and connecting the device to at least one of the column electrodes to supply electrical potential(s) thereto, wherein the display displays desired images.
 37. The apparatus of claim 36, wherein said power supply includes a voltage regulator, a comparator and a current source.
 38. The apparatus of claim 36, wherein said circuit connects the device alternately to the power supply and the at least one column electrode.
 39. The apparatus of claim 36, said apparatus comprising a first and a second electrical energy storage device, wherein said circuit connects the power supply and the at least one column electrode to the two energy storage devices in alternate row scanning cycles.
 40. The apparatus of claim 39, wherein said circuit connects the power supply and the at least one column electrode to the two energy storage devices according to a switching timing waveform that is delayed relative to said row scanning cycles, so that at least one of the energy storage devices is connected to the at least one column electrode during a portion of a row scanning cycle, and the remaining energy storage device is connected to the at least one column electrode during another portion of such row scanning cycle.
 41. The apparatus of claim 40, wherein said time delay is such that a major portion of the energy of the first energy storage device is transferred to the at least one column electrode during a beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to close to a target value, and a minor portion of the energy of the second energy storage device is transferred to the at least one column electrode during but after the beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to substantially the target value.
 42. The apparatus of claim 40, wherein said switching timing waveform is such that during a portion of such row scanning cycle, both the first and the second energy storage devices are connected to the at least one column electrode.
 43. The apparatus of claim 39, wherein the first energy storage drives the voltage of said at least one column electrode to close to a target value during a beginning portion of a row scanning cycle, and the second energy storage device drives the voltage of said at least one column electrode to substantially the target value during but after the beginning portion of such row scanning cycle.
 44. The apparatus of claim 43, wherein a major portion of the energy of the first energy storage device is transferred to the at least one column electrode during a beginning portion of a row scanning cycle, thereby driving the voltage of said at least one column electrode to close to a target value, and a minor portion of the energy of the second energy storage device is transferred to the at least one column electrode during but after the beginning portion of such row scanning cycle, thereby driving the voltage of said at least one column electrode to substantially the target value.
 45. The apparatus of claim 39, wherein during a portion of a row scanning cycle, both the first and the second energy storage devices are connected to the at least one column electrode. 